Thermal Imaging Training: “Chestnuts Roasting on an Open Fire”

Although you are reading this article in December, I’m writing it in October. When I stopped by a local grocery store this morning, I couldn’t help but notice that Christmas music was already playing overhead. Christmas music in October strikes me...


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Although you are reading this article in December, I’m writing it in October. When I stopped by a local grocery store this morning, I couldn’t help but notice that Christmas music was already playing overhead. Christmas music in October strikes me particularly wrong. It’s not that I’m some type of Scrooge; I just think we should celebrate holidays in order: Thanksgiving first, then Christmas.

As I pondered the situation, the words of “The Christmas Song” inevitably soaked into my brain – “Chestnuts roasting on an open fire.” This may conjure warm memories of standing close to a campfire, trying to stay warm against winter’s chill, but here in Kentucky, it is not all that cold in October. In fact, the high today is supposed to be in the low 70s, so rather than mental images of staying warm, I began to conjure mental images of “seeing” warm. And since you will be reading this in December, when many of you will be fighting off winter’s chill, I decided that rather than resist, I would encourage. Go light a fire. Grab the chestnuts. Stay warm – but bring your thermal imager, as there is much to be learned!

I’m sure I don’t have to tell you how to start a fire, so I will skip that, but you will need the following:

  • • Firewood. You will need enough split, dried firewood to keep a hot bed of coals, but you will also want a few green pieces as well – wood that has been freshly cut or not split and dried.
  • • Kindling. This can be paper, small sticks or cardboard in various densities and sizes.
  • • A wooden box. You might have to make this one, but a wooden box configured so that you have a B, C and D wall as well as a ceiling – no floor and no A wall. The three walls should be slightly longer than the base of the fire you will create and approximately twice the height of the firewood. Thicker wood will last longer, so half-inch or three-quarter-inch plywood would be ideal. If you want to get really crazy, cut a hole in the center of the “ceiling” that represents about one-fourth of the total size of the ceiling. Cut another piece of plywood the same size as your ceiling, but with a hole in the center that represents about one-tenth of the total ceiling size and place it on top of the box so that the smaller hole is centered over the larger hole. Finally, cut a piece of plywood to cover the smaller hole. The hole-cutting is not necessary, but will extend training if you have the time.
  • • Your thermal imager. Make sure you are familiar with the temperature ratings associated with the colorization scheme your imager uses. If you don’t know, reference the owner’s manual, the manufacturer’s website or simply call the manufacturer and ask.
  • • Chestnuts. What fun would it be if you couldn’t eat while you train?

As soon as you set the fire, begin to observe with the thermal imager. Look at the difference between what you see with your eyes and what the imager sees. The two images will look very different. In the incipient stages of your campfire, the flames are likely to be large and very bright to your eyes; however, the thermal imager will view them as fairly small.

The main difference is that your eyes see light and the thermal imager does not, so while you perceive the flame to be large based on the amount of light entering your eye, the thermal imager perceives the flame as smaller as it can only assess the heat associated with the flame and cannot see the light. The secondary difference is that, in the incipient stages of a fire, lighter-weight combustibles are usually the first to pyrolize and become consumed. Lighter fuels don’t generate as much heat as heavier fuels do, but they pyrolize rapidly resulting in a large flame with relatively little heat. This is why you typically use kindling to get a campfire started.

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